Gallium nitride (GaN) light-emitting devices have low light extraction efficiency due to large refraction index difference between GaN and air. The light emitted from the active layer in a GaN-based light-emitting device is significantly trapped in the GaN-based layer. Therefore, a serious total internal reflection and re-absorption phenomena would be happened in the GaN-based light-emitting devices. It is well known that a typical p-type GaN (from 300 to 5000 Å thick) layer has a low current conductivity and poor current spreading ability which might cause current crowding phenomenon due to the acceptor (magnesium, Mg) in the GaN crystal having poor activated ability.
LEDs which are based on GaN compounds generally include a transparent insulating substrate, i.e. a sapphire substrate. With a transparent substrate, emission light from the active layer may be utilized from either the substrate or from the opposite end of the LED which is called the “window”. However, a good window layer has some unique properties, including a good conductivity and less optical absorption. According to those properties, the insulating sapphire substrate can not be a good window layer since the GaN-based LED structure grown on sapphire substrate is a horizontal device, which will decrease its emission efficacy.
The amount of light generated by an LED is dependent on the distribution of the energizing current across the face of the light-emitting region (upside emission area). It is well known that the current flowing between the electrodes tends to concentrate in a favored path directly under the electrode. This tends to activate corresponding favored portions of the light-emitting region to the exclusion of portions which fall outside the favored path. Further since such favored paths fall under the opaque electrode, the generated light reaching the electrode is lost.
Alternatively, a conductive transparent material, Indium Tin Oxide (ITO) is studied and applied as a current spreading layer. As disclosed in U.S. Pat. No. 5,481,122, a GaP layer is replaced by an ITO layer to serve as a current spreading layer. The optical transmission coefficient of ITO layer is about 90% in the visible range. The electrical resistivity of n-type ITO is 100 times smaller than that of p-type GaP. However, a Schottky contact is formed between the ITO layer and p-type contact layer. It might degrade the optical performance of the LEDs due to the input power consumption.
EP0434233 discloses a light-emitting diode having improved brightness which has a semiconductor substrate underlying active P-N junction layers of AlGaInP for emitting light. A transparent window layer of semiconductor different from AlGaInP overlies the active layers and has a lower electrical resistivity than the active layers and a bandgap greater than the bandgap of the active layers, for minimizing current crowding from a metal electrical contact over the transparent window layer. A layer of lattice mismatched GaP is then grown on the active layers with the GaP having a bandgap greater than the bandgap of the active layers so that it is transparent to light emitted by the LED. The GaAs temporary substrate is then selectively etched away so that the GaP acts as a transparent substrate. A transparent window layer is epitaxially grown over the active layers on the face previously adjacent to the GaAs substrate. The layer of lattice mismatched GaP which is used as a current spreading layer is transparent to light emitted from the active layer, because it has a band-gap which is greater than the band-gap of the active layer. The resistivity of the material of such layer is low owing to the heavy elements doping, and thus the current flowing vertically through the device spreads out laterally over substantially the entire active region.
A disadvantage of this arrangement is that the current spreading layer has to be very thick (up to 50 μm) in order for the current to spread out sufficiently as it flows vertically through the wafer. Such thick layers of bulk semiconductor material are difficult, expensive and time consuming to grow. It will be appreciated that LEDs are commonly used components in electronic apparatus, and thus their price needs to be cost down. Hence, high-intensity LEDs are unreliable owing to threading dislocations and stacking faults which occur near the lattice mismatched AlGaInP and GaP crystal interface, as a result of strain-relaxation in the thick layer.
The present invention provides a solution to the problems mentioned above. Via the invention, electrical injection efficiency and light extraction efficiency of a light-emitting device can be enhanced.